1. Field of the Invention
The present invention relates to a method of injection molding multi-ply products particularly using a plastic heat-insulating barrier layer between an injection cavity mold and a hot-runner nozzle.
2. Description of the Related Art
Exemplifying a stretch-blow molded bottle, it is required to have barrier properties against oxygen and carbon dioxide, a heat resistance and others. To meet these requirements, the preform of a bottle to be blow molded may be injection molded into a three-layered structure which includes inner and outer layers of structural resin material such as polyethylene terephthalate and an intermediate layer interposed therebetween and made of another resin material which satisfies the aforementioned requirements. Many hot-runner molds used for injection molding such a multi-ply product are known as from Japanese Utility Model Publication No. She 63-37222, Japanese Patent Laid-Open No. Sho 63-107525, Japanese Patent Laid-Open No. Sho 63-221024, Japanese Patent Laid-Open No. Sho 63-239022 and so on.
Such a multi-ply molding is carried out by charging different molten plastic materials into a space between an injection cavity mold and a core mold through the nozzles of a multi-ply molding hot-runner mold.
When performs are generally to be injection molded without limiting to the multi-ply molding, it is required to insulate between the injection cavity mold and the hot-runner nozzle. The hot-runner nozzle is at a raised temperature near the molding temperature of plastic material, for example, two hundred and several tens .degree.C while tile injection cavity mold is only about 10.degree. C.--about 80.degree. C. Such a substantial temperature difference between the injection cavity mold and the hot-runner nozzle will cool the hot-runner nozzle to tile undesirably low temperature, resulting in reduction of the temperature at the inject ion port of the hot-runner nozzle. Thus, the plastic material being charged will be cooled and solidified at the injection port, leading to clogging therein.
In order to overcome such a problem, it is known to dispose an insulation material between the injection cavity mold and tile hot-runner nozzle. Since the property elf such an insulation material degrades due to heat deterioration with the passage of time, it must be frequently replaced by a new insulation material. The replacement of insulation not only requires a troublesome dis-assembly of mold, but also very reduces the efficiency of production since an increased time is required to heat up the mold after it has been once cooled down.
It is also known to form an insulating barrier layer from the plastic material being charged into the injection cavity mold. This barrier layer is superior to the aforementioned insulation in that the barrier layer will not be affected by heat deterioration for a relatively prolonged period.
When a plurality of different plastic materials are used to form a preform of multi-ply cross-section structure, one must note that each of-these plastic materials has its inherent heat-resistant temperature. If the heat-resistant temperature of a plastic material used to form an insulating barrier layer is lower than those of the other plastic materials used together, that plastic material may be thermally deteriorated for a relatively short time period, as in the insulation material. In addition, the plastic material reduced in viscosity due to heat deterioration may be injected into tile injection cavity mold to mold an unacceptable product. For example, ethylene-vinyl alcohol copolymer (hereinafter referred to EV-OH) effective to form a barrier layer has its molding-temperature about 50.degree. C. lower than that of polyethylene terephthalate (hereinafter referred to PET). This is substantially similar to the difference or heat-resistant temperature between these plastic materials. When such two difference plastic materials are to be injection molded into a product and if no consideration is taken with respect to the formation of an insulating barrier layer, these difference plastic materials will be combined at the injection port of the hot-runner nozzle and then flow into a region of forming the barrier layer therethrough. The barrier layer may be partially or at the worst entirely formed of EV-OH. The barrier layer thus formed will relatively easily deteriorate due to heat from the other plastic materials having higher molding temperatures.
There are also plastic materials which will thermally deteriorate when they are subjected to heat for a prolonged time period. In order to avoid the above problems, plastic materials used to form the insulating barrier layer should be selected carefully.